Humans have a great love of flowers, but it’s not unconditional. Tens of millions of people suffer sneezing, coughing, and watery eyes from allergies to certain types of pollen.

What exactly causes so many people so much misery? In plants, the haploid cells produced by meiosis are called spores. Spores produced in the anthers of flowers are called microspores, and spores produced in the ovules are called megaspores. As the anther grows, it forms four chambers—sometimes called “spore sacs”—and each is filled with diploid cells called microspore mother cells. (To see the four spore sacs, slice the anther of a flower in half.) The microspore mother cells divide by meiosis, each producing four haploid microspores. The microspores then quickly divide by mitosis, forming two-celled grains of pollen with very complex, water-tight, sticky surfaces. This two-celled structure is the pollen grain, a reproductive packet containing two haploid cells (FIGURE 18-11). One of the cells will eventually grow to form a pollen tube (FIGURE 18-12), aiding in fertilization. The other will divide once to produce two sperm cells. But those steps don’t usually occur until the pollen grain has made its way from the anther into the environment and come to rest on a stigma somewhere.

Figure 18-11The anther of a flower produces pollen grains containing the male gametes.

Figure 18-12A pollen grain contains two haploid cells. One forms a tube used as a conduit for fertilization. The other cell divides once and forms two sperm cells.

Figure 18-13Pollen grains can cause misery for those who suffer with allergies.

Figure 18-14The pollen on this nose hair may be flushed from the body by mucus (or a sneeze!).

Why does pollen cause allergy attacks in so many people? The trouble begins each spring and summer, as plants release billions and billions of tiny pollen grains into the air. Proteins project from the cell wall of every one of these pollen grains, increasing the pollen’s stickiness. Inevitably, we breathe in some pollen grains, and our bodies immediately detect the pollen’s surface proteins and identify them as foreign invaders (FIGURE 18-13). In response to this invasion, our immune system attacks and tries to flush the pollen from our bodies. Mucus on nose hairs helps move the pollen outside the nose (FIGURE 18-14). However, sometimes the response is extreme. The runny nose, sneezing, coughing, and congestion common to pollen allergies are all manifestations of the body’s over-reaction.